Prior to ignition, gas discharge lamps present a substantially higher impedance to the drive circuits of an electronic ballast than they do during steady-state operation. The removal of one or more lamps from the ballast would similarly result in an increased impedance level. Such high impedance levels could result in the production of dangerously high voltages within the electronic ballast. Such voltages, particularly if they are sustained at a high level for any period of time, could result in breakdown and destruction of solid-state components, or the electrocution of an individual attempting to change a lamp.
It is therefore an object of the present invention to avoid excessively high voltages in an electronic ballast for discharge lamps when a lamp is removed from the ballast, or during start-up, prior to the firing of the lamps.
It is also an object of the present invention to provide a solid-state ballast which is reliable and convenient in use, yet relatively simple and inexpensive in construction.
In accordance with an illustrative embodiment demonstrating objects and features of the present invention, a solid-state ballast for discharge lamps includes a signal controlled oscillator that provides the operating signal for a resonantly driven lamp load. The voltage across the load is sensed and a signal representing this voltage is fed back to a circuit which controls the oscillator so as to limit the maximum load voltage during initial start-up and when a lamp is removed. In accordance with a second embodiment, the voltage is cycled between a low level and a level high enough to ignite the lamp, so long as the lamp is not ignited.